Shear stress or skin friction measurement is a crucial topic in fluid mechanics. It not only provides insights into complex flow phenomena which help the design of watercraft or aircraft, but also may serve as a feedback for flow-control. There are two broad classes of methods for shear stress measurement: indirect measurement and direct measurement. Indirect measure methods usually rely on theoretical models and associated assumptions, which are inaccurate if the flow is complex. Direct measurement methods do not require prior knowledge of the flow conditions and usually employ a floating element, which displaces due to shear stress.
Among the direct measurement methods, micro-electro-mechanical-systems (MEMS)-based shear stress sensors are promising for shear stress measurement because of its compactness, monolithic structure, high sensitivity and accuracy, and high spatial and temporal resolution. However, one cannot obtain the floating element of a wide range of sizes (e.g., tens of centimeters in length) because the fabrication is limited by the size of silicon wafer used for MEMS fabrication. Also, changing sizes of the floating element requires a new set of photolithography masks. Overall, MEMS devices are too expensive for small-batch and customized orders. Moreover, MEMS sensors are usually not robust when subject to water or particle impingement; and are thus constrained by the packaging technique. Also, MEMS devices usually use silicon, which is brittle and easily broken when sample surfaces are frequently mounted thereon.
Among the direct measurement methods, the single-pivot shear stress sensor is a well-known design commonly used in practice. Unlike MEMS sensor and this invention, the single-pivot shear stress sensor is usually assembled of parts instead of monolithically fabricated. The need to assemble the device increases the chance of floating element misalignment and also increases the cost. However, it is made mostly of metals and robust for experimentation and handling. A shear sensor of a direct measurement type made of a tough material such as a metal (as opposed to a brittle material like silicon) yet based on monolithic construction for high performance is highly desired. A key feature of the shear sensing unit in this invention is the relatively large floating element with microscale features made monolithically of a non-brittle material.